It is known to use a second printer to print additional indicia on documents having previously received printing from a primary or host printer. The additional indicia can be, for example, color highlights printed onto black and white documents exiting high speed electrographic or xerographic printers. The preprinted documents are typically formed onto a continuous web of a print receiving material. The additional indicia are added onto the preprinted document at particular positions. Accurately positioning of the additional indicia relative to the preprinted materials is an important requirement of secondary printing. Additionally, for many printing environments, this accurate positioning of the additional indicia must be accomplished at a relatively high throughput rate in order to match or be synchronized with the throughput rate of the host printer. An example of a printing apparatus for the addition of color indicia to a pre-printed document is disclosed in U.S. patent application Ser. No. 08/552,798, entitled "A Printer Assembly", which is incorporated by reference herein.
The secondary printer senses registration features formed in or on the web. The registration features allow tracking the web movement for accurate positioning of the additional indicia. These registration features can include top of form or registration marks printed by the host printer, or tractor feed holes positioned on the longitudinal edges of the web. The secondary printer senses a registration mark and synchronizes the print positioning system from the position of the registration marks. The printing positioning system is re-synchronized at the sensing of each new registration feature to continue to provide accurate positioning of the additional indicia.
Print positioning errors can arise when the registration features are damaged or missing. Furthermore, the print positioning system can interpret stray marks or other inconsistencies on the web as actual registration features. Re-synchronization of the print positioning system from these false registration features further degrades print position performance.
Furthermore, positioning errors can arise from the physical properties of the web. Print receiving web materials, most typically paper, can exhibit elasticity due to web tension, moisture content and thermal factors that affect actual document length, and therefore, print positioning. This elasticity of the web medium can result in either stretching or shrinking of the web in the process direction therefore resulting in variations of the distance between registration features.
The determination of web position must not only be performed accurately, but in real time at a rate commensurate with the high document output rates of many host printers. Prior print positioning systems are often incapable of implementation at the necessary throughput rates while simultaneously maintaining a high degree of accuracy for the print assembly.